Electromagnetic (“EM”) survey technology, referred to below as “EM-survey technology,” has been commercially used for locating hydrocarbon-rich subterranean features for less than 15 years. Marine controlled-source-EM (“CSEM”) survey techniques and multi-transient-EM (“MTEM”) survey techniques are two examples of EM-survey technology. CSEM-survey techniques employ generation of primary propagating time-varying EM fields using various EM sources, including towed bipole antennas, stationary bipoles, and other mobile and stationary EM sources. The primary propagating time-varying EM field extends downward into the subterranean environment, where the field induces secondary currents. The induced secondary currents, in turn, generate a secondary propagating time-varying EM field that is sensed, at various locations distributed across a relatively large area, in order to detect non-uniformities in the secondary EM field resulting from non-uniform electrical resistance in various features within the subterranean environment. Hydrocarbons and hydrocarbon-saturated rocks and sediments have much higher resistivities than water and water-saturated rocks and sediments. High-resistance subterranean pooled hydrocarbons and hydrocarbon-saturated rocks and sediments result in non-uniform distribution of secondary current paths and concentration of electrical field lines in conductive portions of the subterranean environment above the pooled hydrocarbons and hydrocarbon-saturated rocks and sediments. By taking multiple measurements across a wide area for each of many different bipole-antenna-transmitter locations, digitally encoded data sets are generated and stored in data-storage systems, which are subsequently computationally processed in order to provide indications of the longitudinal and latitudinal positions and depths of potential hydrocarbon-rich subterranean features. In many cases, three-dimensional plots, maps, or images, of the subterranean environment are generated as a result of these data-processing operations. The maps and images produced from CSEM-survey data can be used alone or in combination with maps and images produced by other methods, including marine exploration seismic methods, to locate subterranean hydrocarbon sources prior to undertaking the expense of marine-drilling operations to recover liquid hydrocarbon from subterranean sources.
MTEM-survey techniques are generally a time-domain EM technology, and are applicable to both land and marine surveying. While CSEM-survey sources generally produce continuous harmonic signals with a fundamental frequency and odd harmonics, MTEM-survey sources generally produce pseudo-random binary sequences (“PRBS”) with a broad range of frequencies.
In the following discussion, the methods and systems to which the current application is directed are described in the context of CSEM-survey techniques. However, the methods and systems to which the current application is directed may be applied to MTEM-survey data and to data generated by other EM-survey techniques. To be clear, the phrase “EM-survey technique” refers to any of various techniques encompassed by EM-survey technology, including CSEM-survey and MTEM-survey techniques, the phrase “EM sensor” refers to any of various types of EM sensors used in various EM-survey techniques, including electric-field-strength sensors, electric-current-flux sensors, magnetic-field-strength sensors, and other such sensors, and the phrase “EM-sensor data” refers to data collected by EM sensors deployed in any of various EM-survey techniques, including CSEM-survey and MTEM-survey techniques. EM-survey techniques employ towed, or mobile, EM sources and EM sensors, stationary EM sources and EM sensors, or a combination of stationary and mobile EM sources and EM sensors of various types for producing a variety of different types of primary propagating EM fields, or primary EM signals, and detecting secondary propagating EM fields, or secondary EM signals, induced by the primary EM signals.
The raw CSEM-survey data collected during a CSEM survey are generally processed in numerous steps in order to create plots, maps, and/or images of a subterranean environment. In one initial step, described below, collected CSEM-survey data sets are deconvolved with respect to the EM signals used to elicit the CSEM-survey data in order to generate one or more earth-response curves, also referred to as “earth responses.” In general, the earth-response curves do not correspond to simple, closed-form parameterized expressions, but are instead relatively complex response signals resulting from many different signal-propagation, signal-attenuation, and signal-modification characteristics of the complex subterranean environment and of the aqueous environment overlaying the subterranean environment. Currently, the secondary propagating time-varying EM field generated from the primary propagating time-varying EM field is sampled uniformly in time or frequency. However, uniform sampling generally results in sparsely sampling certain portions of the response signal and oversampling other portions of the response signal, resulting in uneven accuracy with which the earth-response function or curve is extracted from the raw CSEM-survey data. The limited accuracy of the extracted earth-response curves may propagate through, and be amplified in, subsequent intermediate and final-stage data processing, resulting in significant errors and lower-than-desired resolution in the plots, maps, and images produced as a product of CSEM-survey data analysis.